With the advent of the tank in World War One, antitank rifles became a priority for many countries, to provide infantry with some weapon to counter the new armored threat. The best known example of these (and the only one to see significant production before the end of WWI) was the German Tankgewehr, any captured Tankgewehrs would form the basis for the US development of the .50 BMG cartridge.
This experimental Winchester rifle, made in late 1918, was part of that development. It is the only known surviving example of its type, and suffered a catastrophic failure during testing, as the receiver is cracked completely in half. The mechanism, however, is quite interesting. It uses a 1911-styled pistol grip as the bolt handle (similar to the Czech SS41 antitank rifle that would come decades later). It was fitted with a relatively large detachable box magazine and a mounting point for a telescopic sight.
Thanks to the Cody Firearms Museum for allowing me access to film it!